1 College of Land Science and Technology, China Agricultural University, Beijing 100083, China.
2 School of Geography and Ocean Science, Nanjing University, Nanjing 210023, China.
3 State Key Laboratory of Resources and Environmental Information Systems, Institute of Geographic Science & Nature Resources Research, Chinese Academy of Sciences, Beijing 100101, China.
4 College of Global and Earth System Sciences, Beijing Normal University, Beijing 100875, China. Electronic address: zychen@bnu.edu.cn.